Phytoremediation of Cadmium-Contaminated Soil: The Role of CO2

Writing as background for their study Li et al. (2012) state that "phytoremediation of heavy-metal-contaminated soil is an emerging technology (McGrath and Zhao, 2003; Kramer, 2005)," which process they describe as making use of "pollutant-accumulating plants to extract pollutants from the soil and accumulate them in the harvestable above-ground biomass," citing as examples the work of Kumar et al. (1995), Chaney et al. (1997), McGrath and Zhao (2003) and Robinson et al. (2009).

Against this backdrop, Li et al. grew hyper-Cd-accumulating Sedum alfredii seedlings within controlled-environment chambers that were maintained at either 350 or 800 ppm atmospheric CO2 concentrations for 60 days in pots containing soil with three different levels of cadmium (Cd) contamination - which they categorized as slight, medium and severe (corresponding to 0, 5 and 50 mg Cd added per kg of soil, respectively) - after which the plants were harvested and, along with their biomass, the amounts of Cd sequestered in their shoots and roots were determined, which allowed them to calculate the total amounts of Cd removed from the three sets of soils of the two CO2 treatments.

The four Chinese researchers report that the shoot dry weights of the CO2-enriched plants in the slight, medium and severe Cd contamination treatments were enhanced by 36.7%, 26.7% and 24.6%, respectively, relative to those of the plants growing in ambient air. Likewise, they found that corresponding increases in root dry weights were 52.1%, 36.7% and 35.0%. And, last of all, they determined that the overall phytoextraction efficiency of Cd was increased by 34.2%, 48.3% and 41.3%, respectively, in the slight, medium and severe Cd contamination treatments, when compared with the plants grown in ambient CO2 air.

Given such findings, Li et al. suggest that "the use of elevated CO2 may be a useful way to improve phytoremediation efficiency of Cd-contaminated soil by S. alfredii." And as the air's CO2 content continues its upward climb, it could be argued that the job of the hyper-accumulating plants will gradually become ever more efficient and effective.